The present invention relates to a nondispersive infrared gas analyzer.
Gas analyzers of the type to which the invention pertains use the following principle. A test or sample beam of radiation traverses the substance (measuring gas) to be analyzed, wherein a portion of that radiation is frequency selectively absorbed. The, thus, attenuated radiation is passed on to a receiving cell which, likewise, absorbs radiation at the same frequency as the particular substance to be detected, but the cell content should not have any common absorption band with the other components in the substance to be analyzed. In practice, one uses in the receiver cell a specific quantity of the substance to be detected. Upon absorbing radiation, the receiving cell will undergo a change in temperature, pressure, or volume, and that change is measured as a representation of the concentration of the substance of interest as a portion of the radiation was previously absorbed by the substance to be analyzed.
The German Pat. No. 730,478 discloses such an analyzer in which, moreover, two beams are provided, and the second one traverses a reference cell before reaching a (second) receiving cell. Further, both beams are modulated by a chopper, and the output is generated by comprising electrically, e.g., the pressures in the two receiver cells.
This particular type of analyzer has been practiced with advantage, but its application is somewhat limited. The limitation arises from the requirement that the gas component to be detected should not have a common absorbtion band with the host gas (host gas plus component gas constitute the measuring gas). If there are overlapping bands, errors arise. One has attempted in many ways to eliminate this phenomenon, also termed cross sensitivity.
German printed patent application No. 25 52 165, page 6, mentions the use of interference filters which eliminates those portions in and from the radiation spectrum in the test beams which fall into that overlapping range or ranges. Unfortunately, the loss in useful radiation is a considerable one. Moreover, it is very difficult in many cases to make such a filter which is really effective in the elimination of the overlapping band portions.
U.S. Pat. No. 2,761,067 suggests to eliminate the cross sensitivity by means of using measuring and reference beams of different wavelength and differently long chambers for the sample gas in both beam paths. This approach was found impractical as the reference beam in this case is not really a reference beam, and poses other problems.